Iron cycling in the anoxic cryo-ecosystem of Antarctic Lake Vida
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Iron redox cycling in metal-rich, hypersaline, anoxic brines plays a central role in the biogeochemical evolution of life on Earth, and similar brines with the potential to harbor life are thought to exist elsewhere in the solar system. To investigate iron biogeochemical cycling in a terrestrial analog we determined the iron redox chemistry and isotopic signatures in the cryoencapsulated liquid brines found in frozen Lake Vida, East Antarctica. We used both in situ voltammetry and the spectrophotometric ferrozine method to determine iron speciation in Lake Vida brine (LVBr). Our results show that iron speciation in the anoxic LVBr was, unexpectedly, not free Fe(II). Iron isotope analysis revealed highly depleted values of −2.5‰ for the ferric iron of LVBr that are similar to iron isotopic signatures of Fe(II) produced by dissimilatory iron reduction. The presence of Fe(III) in LVBr therefore indicates dynamic iron redox cycling beyond iron reduction. Furthermore, extremely low δ18O–SO4 2− values (−9.7‰) support microbial iron-sulfur cycling reactions. In combination with evidence for chemodenitrification resulting in iron oxidation, we conclude that coupled abiotic and biotic redox reactions are driving the iron cycle in Lake Vida brine. Our findings challenge the current state of knowledge of anoxic brine chemistry and may serve as an analogue for icy brines found in the outer reaches of the solar system.
KeywordsMcMurdo dry valleys Lake Vida Iron Redox
Financial support for this research is gratefully acknowledged from National Science Foundation Awards ANT-0739681 (to A.E.M.) and ANT-0739698 (to P.T.D. and F.K.). N.E.O. gratefully acknowledges support from the NSF Geobiology and Low Temperature Geochemistry program (Grants 1053432 and 1348935). This work was also funded in part by the DOE Great Lakes Bioenergy Research Center (DOE Office of Science BER DE-FCO2-07ER64494) to N.E.O. Additional support was provided by NASA–NASA Astrobiology Institutes “Icy Worlds” (to A.E.M.) and “Water and Astrobiology” at the University of Hawaii (to B.T.G.). Lastly, the National Science Foundation McMurdo LTER Award (ANT-1041742, to P.T.D.) supported the Lake Hoare voltammetry research.
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